1. Field of Invention
This invention relates to the formation of olefins by thermal cracking of whole crude oil. More particularly, this invention relates to utilizing whole crude oil as a feedstock for an olefin production plant that employs a hydrocarbon cracking process such as steam cracking in a pyrolysis furnace.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
Basically, a hydrocarbon feedstock such as naphtha, gas oil or other fractions of whole crude oil that are produced by distilling or otherwise fractionating whole crude oil, is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. The steam/hydrocarbon mixture is preheated to from about 900 to about 1,000 degrees Fahrenheit (° F. or F), and then enters the reaction zone where it is very quickly heated to a severe hydrocarbon cracking temperature in the range of from about 1,450 to about 1,550 F.
This process is carried out in a pyrolysis furnace (steam cracker) at pressures in the reaction zone ranging from about 10 to about 30 psig. Pyrolysis furnaces have internally thereof a convection section and a radiant section. Preheating is accomplished in the convection section, while severe cracking occurs in the radiant section.
After severe cracking, the effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety, e.g., from one to thirty-five carbon atoms per molecule. These gaseous hydrocarbons can be saturated, monounsaturated, and polyunsaturated, and can be aliphatic, alicyclics, and/or aromatic. The cracked gas also contains significant amounts of molecular hydrogen (hydrogen).
Thus, conventional steam cracking, as carried out in a commercial olefin production plant, employs a fraction of whole crude and totally vaporizes that fraction while thermally cracking same. The cracked product can contain, for example, about 1 weight percent (wt. %) hydrogen, about 10 wt. % methane, about 25 wt. % ethylene, and about 17 wt. % propylene, all wt. % being based on the total weight of said product, with the remainder consisting mostly of other hydrocarbon molecules having from 4 to 35 carbon atoms per molecule.
The cracked product is then further processed in the olefin production plant to produce, as products of the plant, various separate individual streams of high purity such as hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon atoms per molecule, fuel oil, and pyrolysis gasoline. Each separate individual stream aforesaid is a valuable commercial product in its own right. Thus, an olefin production plant currently takes a part (fraction) of a whole crude stream and generates a plurality of separate, valuable products therefrom.
The starting material from which a feedstock for a conventional olefin production plant, as described above, normally has first been subjected to substantial, expensive processing before it reaches that plant. Normally, whole crude is distilled or otherwise fractionated into a plurality of fractions such as gasoline, kerosene, naphtha, gas oil (vacuum or atmospheric) and the like, including a high boiling residuum. Thereafter any of these fractions, other than the residuum, can be passed to an olefin production plant as the feedstock for that plant.
It would be desirable to be able to forego the capital and operating cost of a refinery distillation unit (whole crude processing unit) that processes crude oil to generate a crude oil fraction that serves as feedstock for conventional olefin producing plants. However, the prior art, until recently, taught away from even hydrocarbon cuts (fractions) that have too broad a boiling range distribution. For example, see U.S. Pat. No. 5,817,226 to Lenglet.
Recently, U.S. Pat. No. 6,743,961 issued to Donald H. Powers. This patent relates to cracking whole crude oil by employing a vaporization/mild cracking zone that contains packing. This zone is operated in a manner such that the liquid phase of the whole crude that has not already been vaporized is held in that zone until cracking/vaporization of the more tenacious hydrocarbon liquid components is maximized. This allows only a minimum of solid residue formation which residue remains behind as a deposit on the packing. This residue is later burned off the packing by conventional steam air decoking, ideally during the normal furnace decoking cycle, see column 7, lines 50-58 of that patent. Thus, the second zone 9 of that patent serves as a trap for components, including hydrocarbonaceous materials, of the crude oil feed that cannot be cracked or vaporized under the conditions employed in the process, see column 8, lines 60-64 of that patent.
U.S. patent application Ser. No. 10/244,792, filed Sep. 16, 2002, having common inventorship and assignee with U.S. Pat. No. 6,743,961, is directed to the process disclosed in that patent but which employs a mildly acidic cracking catalyst to drive the overall function of the vaporization/mild cracking unit more toward the mild cracking end of the vaporization (without prior mild cracking)—mild cracking (followed by vaporization) spectrum.
U.S. patent application Ser. No. 10/616,839, filed Jul. 10, 2003, having common inventorship and assignee with U.S. Pat. No. 6,743,961, is directed to the process disclosed in that patent but which removes at least part of the liquid hydrocarbons remaining in the vaporization/mild cracking unit that are not yet vaporized or mildly cracked. These liquid hydrocarbon components of the crude oil feed are drawn from near the bottom of that unit and passed to a separate controlled cavitation device to provide additional cracking energy for those tenacious hydrocarbon components that have previously resisted vaporization and mild cracking. Thus, that invention also seeks to drive the overall process in the vaporization/mild cracking unit more toward the mild cracking end of the vaporization—mild cracking spectrum aforesaid.